"Michael A. Covington" writes:

My students are having the following experience with small permanent-magnet
DC motors:

After some kind of misfortune (overload? nobody knows), the motor starts
drawing much more current than it should. For example, a damaged motor will
draw 1 amp at 3 volts, while the undamaged one will draw only 200 mA at that
voltage. (Normal voltage is much higher, about 10 V.) Both motors turn
easily; the damaged one seems to run fine except for requiring excessive
current; and both have a resistance of about 1.5 ohms measured with an
ohmmeter.

Have you checked resistance and you rotate the rotor to check all
windings?

This has happened to several motors.

Are the permanent magnets getting demagnetized? How? Or what else could be
going on? My electronics background is analog and digital but not much
about motors!

That's a possibility. If you put much more current through the motor
than it expects, this could indeed happen. In any case, it's a simple
matter to compare the magnet strength of a good and bad motor. There
will be an obvious difference. However, I'd expect the rpms for a
given voltage to be different as well.

Another simpler possibility is that there is conducting "crud" on the
commutator. This is a common problem with the motors in CD players
and other consumer electronics. However, your test of the resistance
would seem to rule this out.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
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